The European Telecommunications Standard Institute-Digital Mobile Radio (ETSI-DMR) is a direct digital replacement for analog Private Mobile Radio (PMR). DMR is a scalable system that can be used in unlicensed mode (in a 446.1 to 446.2 MHz band), and in licensed mode, subject to national frequency planning ETSI standards or specifications referred to herein may be obtained by contacting ETSI at ETSI Secretariat, 650, route des Lucioles, 06921 Sophia-Antipolis Cedex, FRANCE. DMR provides improved range, higher data rates, more efficient use of spectrum, and improved battery life over prior direct mode protocols. Features supported include fast call set-up, calls to groups and individuals, and short data and packet data calls. Supported communications modes include individual calls, group calls, and broadcast calls provided via a direct communication mode among the radios operating within the network. Other direct mode functions such as emergency calls, priority calls, full duplex communications, short data messages and Internet Protocol (IP)-packet data transmissions are also supported.
Direct mode, more generally, is a mode of operation where radios may communicate within a network without the assistance of one or more infrastructure equipment (e.g., base stations or repeaters). A radio, as used herein, can be mobile and/or fixed end equipment that is used to obtain direct mode communications services. Direct mode operation is contrasted with a conventional repeater mode, which is a mode of operation where radios communicate with one another through infrastructure equipment such as a repeater or base station. Direct mode, therefore, can provide a more efficient and less costly communication system operation than repeater mode operation.
The ETSI-DMR standard provides for 12.5 Kilohertz (KHz) operation in direct mode (systems that exclusively communicate without a repeater). The 12.5 KHz operation refers to 12.5 KHz spectral efficiency in which there are two communication paths per 12.5 KHz of radio frequency (RF) spectrum. The 12.5 direct mode utilizes 27.5 millisecond (msec) pulsed (every 60 msec) radio transmissions on each of the logical channels. In the 12.5 direct mode of operation, radios transmit asynchronously and radios within range of the transmission synchronize themselves to that transmission for the purposes of receiving the transmission, but any transmissions in response to the first transmission are transmitted asynchronously.
Other direct mode protocols, perhaps consistent with the Project 25 (P25) standard defined by the Association of Public Safety Communications Officials International (APCO) and standardized under the Telecommunications Industry Association (TIA), or with the terrestrial trunked radio (TETRA) standard also defined by the ETSI, may operate in a similar manner and may be used in addition to or in place of the DMR protocol. Communications in accordance with any one or more of these direct mode communication standards, or other standards, may take place over physical channels in accordance with one or more of a TDMA (time division multiple access), FDMA (frequency divisional multiple access), or CDMA (code division multiple access) protocol.
In prior analog direct mode communications systems, two or more simultaneous direct mode transmissions can be mixed at the RF level and the recipient thus able to hear the mixture of all the conversations. However, in a digital conventional direct mode consistent with any one of the foregoing mentioned standards, two or more sources transmitting simultaneously would corrupt both of the digital transmissions and, as a result, the recipients would not hear any conversation. Even worse, the simultaneously transmitting digital radio users would not be aware of the corrupted transmission and may (incorrectly) assume that the transmissions reached their respective targeted group of recipient digital direct mode radios. This issue is further exacerbated in digital direct mode group calls, where the number of digital direct mode radios in the group (e.g., more than five) increases the odds that simultaneous transmissions would conflict.
Accordingly, there is a need to provide an improved call collision arbitration process and apparatus in digital conventional direct mode communication systems in order to improve the robustness of the communications systems and ensure that digital transmissions are not corrupted by multiple simultaneous direct mode digital transmissions.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.